Optical transmission system, optical transmitter for optical transmission system, and optical receiver for optical transmission system

ABSTRACT

An optical transmission system is provided in which the optimum operating point of a Mach-Zehnder interferometer, matched to the optical frequency of the light source on the transmitting side, can be set. The optical receiver ( 2 ) has an infinitesimal-modulated signal component detection circuit ( 222 ), which uses the signal train output from a balanced detection circuit ( 221 ) to detect the infinitesimal-modulated signal component applied to the phase adjustment terminal ( 201 ) of an MZI ( 200 ) by an infinitesimal-modulated signal oscillation circuit ( 224 ); a synchronous detection circuit ( 223 ), which synchronously detects the infinitesimal-modulated signals output from the infinitesimal-modulated signal component detection circuit ( 222 ) and infinitesimal-modulated signal oscillation circuit ( 224 ) and detects the error signal component arising from the shift between the optical signal carrier frequency and the optical frequency characteristic of the MZI ( 200 ); and a controller ( 207 ), which outputs a control signal to adjust the phase difference between two split optical signals output from the MZI ( 200 ) so as to correct the shift amount.

TECHNICAL FIELD

This invention relates to an optical transmission system, opticaltransmitter in an optical transmission system, and optical receiver inan optical transmission system, to which a DPSK-DD scheme is applied.

Priority is claimed on Japanese Patent Application No. 2004-76746, filedon Mar. 17, 2004, the contents of which are incorporated herein byreference.

BACKGROUND ART

The arrival of the age of broadband transmission has been accompanied bydemands for optical transmission systems with ever-greater capacities.It is becoming comparatively easier to achieve large capacities throughwavelength division multiplex (WDM) technology, but there has also beenmuch study of measures to raise the bit rate per wavelength. This isbecause by raising the bit rate per wavelength, device costs can bereduced, device sizes can be decreased, and power consumption can belowered, enabling reductions in the initial costs and running costs foroverall systems.

Electrical circuits which realize 40 Gbit/s/CH have already reached thestage of commercialization. In the WDM transmission of such high-speedoptical signals, limitations on the distance of transmission due tochromatic dispersion, limits to the power input to fibers arising fromfiber nonlinearity, and other problems arise. In particular, in recentyears there has been much study of Differential Phase ShiftKeying-Direct Detection (DPSK-DD) schemes as a means of coping withfiber nonlinearity.

Further, WDM transmission technology employing RZ (Return-to-Zero)-DPSKschemes and CS (Carrier Suppressed) RZ-DPSK schemes, which are stillmore resistant to nonlinearity, is also being studied. It is said thatcompared with the NRZ codes (Non-Return-to-Zero codes) used inconventional optical transmission systems, RZ codes are better able toaccommodate input power limitations.

In a receiver employing DPSK-DD schemes (including RZ-DPSK, CSRZ-DPSK,and other RZ-type DPSK-DD schemes), a photodetector is used for directdetection after conversion into intensity-modulated codes ofphase-modulated signals, using a demodulator such as Mach-Zehnderinterferometer or similar. At this time, by using a double-balancedreceiver, differential photo-detection is possible, and thediscrimination sensitivity is improved by 3 dB compared with cases inwhich intensity-modulated signals are directly detected using a singlephotodetector; hence double-balanced receivers are generally used as thephotodetector.

In order to use a Mach-Zehnder interferometer to demodulatephase-modulated signals into intensity-modulated signals, the pathdifference between the two paths of the Mach-Zehnder interferometer mustbe controlled at the wavelength level to follow fluctuations in thesignal light wavelength. Methods to execute this control include, forexample, a method in which the output level of the balancedphotodetector is detected and a phase shifter provided in one arm of theinterferometer is controlled such that a constant output level isobtained, as explained in Patent Reference 1.

As a Mach-Zehnder interferometer, an optical waveguide-type devicefabricated on a PLC (Planar Lightwave Circuit) is marketed commercially.As the method of control of the path difference, it is possible toeither control the substrate temperature (amount of change in pass band:1.4 GHz/° C.), or to execute control by heating using a heater providedon both arms (amount of phase change: 1.33π/W).

Patent Reference 1: Japanese Unexamined Patent Application, FirstPublication No. S63-52530

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, in the “receiver for coherent optical communication” explainedin Japanese Unexamined Patent Application, First Publication No.S63-52530, because the optimum point of the phase shifter is thedetection signal level maximum value, even if the absolute value of theshift between the signal light wavelength and the pass band of theinterferometer can be detected, the direction of the shift cannot bedetected. This has been one issue in the prior art requiring study.

When applying such methods to WDM transmission systems, because the WDMwavelength spacing and the repetition frequency of the Mach-Zehnderinterferometer generally do not match, the path difference in theMach-Zehnder interferometer must be controlled. (Expressed in terms ofthe frequency domain, the pass band wavelength of the Mach-Zehnderinterferometer must be controlled.) When the signal rate is high, thecontrolled range broadens. For example, for 40 Gbit/s signals, therepetition frequency of the Mach-Zehnder interferometer is 40 GHz, andso the difference between the oscillation wavelength and the pass bandof the Mach-Zehnder interferometer is at maximum 20 GHz. If theMach-Zehnder interferometer is on a PLC, and the pass band is controlledthrough the substrate temperature, then the temperature must be variedby approximately 15° C., so that a large amount of power must beconsumed. This is a second issue requiring study.

The present invention was devised in light of the above circumstances,and has as an object the provision of an optical transmission system,optical transmitter for an optical transmission system, and opticalreceiver for an optical transmission system enabling setting of theoptimum operating point of the Mach-Zehnder interferometer, whichmatches the optical frequency of the light source on in the transmittingside.

Means for Solving the Problem

In order to attain the above objects, an optical transmission system ofthis invention comprises: an optical transmitter which outputsdifferential-encoded phase-modulated light; and an optical receiverwhich detects the phase-modulated light and performs demodulation,wherein the optical transmitter comprises: an encoder which converts NRZcode input signals into NRZ-I code signals; and a phase modulator which,for marks and spaces encoded by the encoder, outputs phase-modulatedlight with a phase deviation Δφ imparted over a range 0≦Δφ≦π, theoptical receiver comprises: a Mach-Zehnder interferometer withphase-adjustment terminal to set a phase difference between twointerfering signals, which splits the phase-modulated light which hasbeen received into two signal light beams, delays one of the splitsignal light beams by one bit, and causes the two signal light beams tointerfere to effect conversion into intensity-modulated light; abalanced detection circuit which performs photoelectric conversion ofsignal light from two output ports of the Mach-Zehnder interferometer,and outputs a difference in converted electrical signals; alow-frequency signal generation circuit which applies a firstlow-frequency signal at frequency f1 to the phase-adjustment terminal ofthe Mach-Zehnder interferometer; an infinitesimal-modulated signalcomponent detection circuit which detects a second low-frequency signalfrom a signal supplied by the balanced detection circuit; a synchronousdetection circuit which, by synchronous detection of the secondlow-frequency signal output from the infinitesimal-modulated signalcomponent detection circuit using the first low-frequency signal outputfrom the low-frequency signal generation circuit, detects a shift amountand direction of shift between a center wavelength of thephase-modulated light output from the optical transmitter and a passband wavelength of the Mach-Zehnder interferometer; a control circuitwhich outputs a control signal to adjust the phase difference betweenthe two split signal light beams so as to correct the shift amount; anda driver circuit which drives the phase adjustment terminal based on thecontrol signal.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may comprise:an eye-opening monitoring circuit which outputs a signal obtained bymonitoring an eye opening of a signal split from the signal output fromthe balanced detection circuit; and a band-pass filter which passes thesecond low-frequency signal contained in the signal output from theeye-opening monitoring circuit, the synchronous detection circuit maydetect the shift amount and direction based on an output signal of theband-pass filter.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may comprise:a data regeneration circuit which discriminates and regenerates anelectrical signal output from the balanced detection circuit and whichis provided with a code error detection function; an error countmonitoring circuit which outputs a signal obtained by monitoring thenumber of errors output from the data regeneration circuit; and aband-pass filter which passes the second low-frequency signal containedin the signal output from the error count monitoring circuit, thesynchronous detection circuit may detect the shift amount and directionbased on an output signal of the band-pass filter.

In an optical transmission system of this invention, the balanceddetection circuit may comprise an equalizing amplification circuit, theinfinitesimal-modulated signal component detection circuit may comprise:a current consumption monitoring circuit which outputs a signal obtainedby monitoring the current consumption of the equalizing amplificationcircuit; and a band-pass filter which passes the second low-frequencysignal contained in the signal output by the current consumptionmonitoring circuit, the synchronous detection circuit may detect theshift amount and direction based on an output signal of the band-passfilter.

In an optical transmission system of this invention, the balanceddetection circuit may comprise: an optical splitting unit which splitsinto two each of the two output ports of the Mach-Zehnderinterferometer; an optical coupling unit which causes interferencebetween two light beams split by the optical splitting unit; and anoptical detection unit which converts an optical signal output from theoptical coupling unit into an electrical signal, theinfinitesimal-modulated signal component detection circuit may comprisea band-pass filter which passes the second low-frequency signalcontained in the electrical signal output from the optical detectionunit, the synchronous detection circuit may detect the shift amount anddirection based on an output signal of the band-pass filter.

In an optical transmission system of this invention, a free spectralrange of the Mach-Zehnder interferometer may be shifted somewhat fromthe clock rate of a main signal, the infinitesimal-modulated signalcomponent detection circuit may comprise: a first amplifier whichamplifies an optical current of one of photodetectors forming thebalanced optical detection circuit; and a band-pass filter whichextracts a component of the second low-frequency signal from an outputof the first amplifier, the synchronous detection circuit may detect theshift amount and direction based on an output signal of the band-passfilter.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may furthercomprise: a second amplifier which amplifies an optical current of theother of the photodetectors forming the balanced detection circuit; anda subtracter which outputs a difference between an output of the firstamplifier and an output of the second amplifier, the band-pass filtermay extract the component of the second low-frequency signal from anoutput of the subtracter.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may comprise:a clock extraction circuit which extracts a clock signal from a signaltrain output from the balanced detection circuit; and a low-frequencysignal extraction circuit which extracts the second low-frequency signalsuperposed on the clock signal output from the clock extraction circuit,the synchronous detection circuit may detect the shift amount anddirection based on the second low-frequency signal output from thelow-frequency signal extraction circuit.

In an optical transmission system of this invention, the opticaltransmitter may comprise: a clock signal generation circuit whichgenerates a clock signal having the same bit rate as a signal bit rate;and an intensity modulator which performs intensity modulation of thephase-modulated light using the clock signal output by the clock signalgeneration circuit, the balanced detection circuit may comprise: anoptical splitting circuit which splits one of the two output ports ofthe Mach-Zehnder interferometer; and a monitoring photodetectorconnected to the optical splitting circuit, the infinitesimal-modulatedsignal component detection circuit may comprise: a narrow-band amplifierwhich extracts a clock on which the second low-frequency signal issuperposed from intensity-modulated light output from the monitoringphotodetector; and a power detection circuit which extracts the secondlow-frequency signal from the extracted clock, the synchronous detectioncircuit may detect the shift amount and direction based on an outputsignal of the power detection circuit.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may comprise:a data regeneration circuit which discriminates and regenerates anelectrical signal output from the balanced detection circuit; acorrelation detection circuit which detects a correlation between anoutput signal of the data regeneration circuit and a signal beforediscrimination; and a low-frequency signal extraction circuit whichextracts the second low-frequency signal from an output of thecorrelation detection circuit.

In an optical transmission system of this invention, the opticaltransmission system may comprise: an intensity modulation unit whichperforms intensity modulation of the phase-modulated light using asignal at frequency f2 sufficiently high to enable superpositioning ofthe low-frequency signal at frequency f1; and an intensity-modulatedcomponent detection unit which detects an intensity-modulated componentat frequency f2, the infinitesimal-modulated signal component detectioncircuit may extract the second low-frequency signal at frequency f1superposed onto the detected intensity-modulated component at frequencyf2.

In an optical transmission system of this invention, as the intensitymodulation unit, the optical transmitter may comprise an oscillationcircuit which generates a signal at the frequency f2 and performs directintensity modulation of a light source of the optical transmitter.

In an optical transmission system of this invention, as the intensitymodulation unit, the optical receiver may comprise: an oscillationcircuit which generates a signal at the frequency f2; and an intensitymodulator which performs intensity modulation of signal light using thesignal output from the oscillation circuit.

In an optical transmission system of this invention, as the intensitymodulation unit, the optical receiver may comprise: an oscillationcircuit which generates a signal at the frequency f2; and an opticalamplifier connected to the oscillation circuit, and that the gain of theoptical amplifier may be modulated by the oscillation circuit at thefrequency f2.

In an optical transmission system of this invention, as theintensity-modulated component detection unit, the optical receiver maycomprise: an optical splitting circuit which splits one of the twooutput ports of the Mach-Zehnder interferometer; a monitoringphotodetector connected to the optical splitting circuit; and anextraction circuit which extracts the component at the frequency f2 fromintensity-modulated light output from the monitoring photodetector.

In an optical transmission system of this invention, as theintensity-modulated component detection unit, the optical receiver maycomprise: an input level adjustment unit which renders asymmetric theinput levels of the converted intensity-modulated light which is inputto the balanced detection circuit; and an extraction circuit whichextracts the component at the frequency f2 from an output signal of thebalanced detection circuit.

In an optical transmission system of this invention, theinfinitesimal-modulated signal component detection circuit may comprisea data regeneration circuit which discriminates and regenerates anelectrical signal output from the balanced detection circuit, theoptical receiver further may comprise: a logic inversion circuit whichinverts the logic of an output signal of the data regeneration circuitand outputs an inverted signal; a selection unit which selectivelyoutputs either the output signal of the data regeneration circuit or anoutput of the logic inversion circuit according to a prescribed logicspecification signal; and a polarity selection unit which inverts thepolarity of a feedback error signal within the control circuit when theoutput of the logic inversion circuit has been selected, an amount ofcorrection of the shift between the center wavelength of thephase-modulated light output from the optical transmitter and the passband wavelength of the Mach-Zehnder interferometer may be reduced to ½or less of a repetition frequency of the Mach-Zehnder interferometer.

In an optical transmission system of this invention, the opticalreceiver may further comprises: a temperature detection circuit whichdetects the temperature of a substrate of the Mach-Zehnderinterferometer; and a loop open/close switch which turns on and offfeedback control to the Mach-Zehnder interferometer, when thetemperature of the substrate of the Mach-Zehnder interferometer is notwithin an appropriate range, the loop to perform the feedback controlmay be opened, whereas when the temperature of the substrate of theMach-Zehnder interferometer is within the appropriate range, the loopmay be closed to perform the feedback control.

In an optical transmission system of this invention, the control circuitmay further comprises: a lock detection circuit which detects a lockedstate of a loop which performs feedback control to the Mach-Zehnderinterferometer; and a re-locking circuit which re-locks the loop whenthe locked state indicates that the loop is unlocked, when the lockdetection circuit is detecting that the loop is locked, normal feedbackcontrol may be performed, and when the lock detection circuit is notdetecting that the loop is locked, a driving signal applied to the phaseadjustment terminal of the Mach-Zehnder interferometer may be swept, andif the lock detection circuit once again detects that the loop islocked, switching to a state in which the normal feedback control may beperformed.

In an optical transmission system of this invention, the Mach-Zehnderinterferometer may provided with two independent phase adjustmentterminals, and an output of the infinitesimal-modulated signaloscillation circuit may be applied to one of the two phase adjustmentterminals, while a feedback error signal within the control circuit maybe applied to the other of the two phase adjustment terminals.

In an optical transmission system of this invention, the opticalreceiver may comprise: an optical carrier frequency detection unit whichdetects, from received signal light detected by the balanced detectioncircuit, a relative position between an optical carrier frequency and anoptical frequency characteristic of the Mach-Zehnder interferometer; andan offset setting circuit which provides an offset to a feedback errorsignal in the control circuit, a value of the offset of the offsetsetting circuit may be adjusted such that the position of the opticalcarrier frequency matches a peak position or bottom position of theoptical frequency characteristic of the Mach-Zehnder interferometer.

In an optical transmission system of this invention, the opticaltransmitter may comprise: a modulation state control unit which turns onand off modulation of a main signal; and a first control signalcommunication unit which communicates with the optical receiver using acontrol line provided separately from a line for the main signal, theoptical receiver may comprise: an optical carrier frequency detectionunit which detects, from received signal light detected by the balanceddetection circuit, a relative position between an optical carrierfrequency and an optical frequency characteristic of the Mach-Zehnderinterferometer; an offset setting circuit which provides an offset to afeedback error signal in the control circuit; and a second controlsignal communication unit which communicates with the opticaltransmitter using the control line, at the time of startup of theoptical transmission system, the optical transmitter may use themodulation state control unit to turn off modulation of the main signaland transmit only an optical carrier, the optical receiver may use theoptical carrier frequency detection unit to detect the relative positionbetween the frequency of the optical carrier transmitted from theoptical transmitter and the optical frequency characteristic of theMach-Zehnder interferometer, and may adjust the offset of the offsetsetting circuit so as to cause a position of the optical carrierfrequency to match a peak or bottom position of the optical frequencycharacteristic of the Mach-Zehnder interferometer, the optical receivermay send a control signal indicating completion of offset adjustment tothe optical transmitter using the second control signal communicationunit, and, after receiving the control signal, the optical transmittermay turn on modulation of the main signal.

An optical transmitter of a first aspect of the present invention is anoptical transmitter, in an optical transmission system comprising: theoptical transmitter which outputs differential-encoded, phase-modulatedlight; and an optical receiver which detects the phase-modulated lightand performs demodulation, wherein the optical transmitter comprises: anencoder which converts NRZ code input signals into NRZ-I code signals;and a phase modulator which, for marks and spaces encoded by theencoder, outputs phase-modulated light with a phase deviation Δφimparted over a range 0≦Δφ≦π, the optical receiver comprises: aMach-Zehnder interferometer with phase-adjustment terminal to set aphase difference between two interfering signals, which splits thephase-modulated light which has been received into two signal lightbeams, delays one of the split signal light beams by one bit, and causesthe two signal light beams to interfere to effect conversion intointensity-modulated light; and a balanced photodetector which performsphotoelectric conversion of signal light from two output ports of theMach-Zehnder interferometer, and outputs a difference in convertedelectrical signals, the optical transmitter comprises: a clock signalgeneration circuit which generates a clock signal having the same bitrate as a signal bit rate; and an intensity modulator which uses theclock signal output from the clock signal generation circuit to performintensity modulation of the phase-modulated light.

An optical transmitter of a second aspect of the present invention is anoptical transmitter, in an optical transmission system comprising: anoptical transmitter which outputs differential-encoded, phase-modulatedlight; and an optical receiver which detects the phase-modulated lightand performs demodulation, wherein the optical transmitter comprises: anencoder which converts NRZ code input signals into NRZ-I code signals;and a phase modulator which, for marks and spaces encoded by theencoder, outputs phase-modulated light with a phase deviation Δφimparted over a range 0≦Δφ≦π, the optical receiver comprises: aMach-Zehnder interferometer with phase-adjustment terminal to set aphase difference between two interfering signals, which splits thephase-modulated light which has been received into two signal lightbeams, delays one of the split signal light beams by one bit, and causesthe two signal light beams to interfere to effect conversion intointensity-modulated light; and a balanced photodetector which performsphotoelectric conversion of signal light from two output ports of theMach-Zehnder interferometer, and outputs a difference in convertedelectrical signals, the optical transmitter comprises an oscillationcircuit which generates a signal at frequency f2 sufficiently high toenable superpositioning of a low-frequency signal at frequency f1 atwhich a light source of the optical transmitter is directlyintensity-modulated.

An optical receiver of the present invention is an optical receiver, inan optical transmission system comprising: an optical transmitter whichoutputs differential-encoded, phase-modulated light; and the opticalreceiver which detects the phase-modulated light and performsdemodulation, wherein the optical transmitter comprises: an encoderwhich converts NRZ code input signals into NRZ-I code signals; and aphase modulator which, for marks and spaces encoded by the encoder,outputs phase-modulated light with a phase deviation Δφ imparted overthe range 0≦Δφ≦π, the optical receiver comprises: a Mach-Zehnderinterferometer with phase-adjustment terminal to set a phase differencebetween two interfering signals, which splits the phase-modulated lightwhich has been received into two signal light beams, delays one of thesplit signal light beams by one bit, and causes the two signal lightbeams to interfere to effect conversion into intensity-modulated light;a balanced detection circuit which performs photoelectric conversion ofsignal light from two output ports of the Mach-Zehnder interferometer,and outputs a difference in converted electrical signals; alow-frequency signal generation circuit which applies a firstlow-frequency signal at frequency f1 to the phase-adjustment terminal ofthe Mach-Zehnder interferometer; an infinitesimal-modulated signalcomponent detection circuit which detects a second low-frequency signalfrom a signal supplied by the balanced detection circuit; a synchronousdetection circuit which detects a shift amount and direction of shiftbetween a center wavelength of the phase-modulated light output from theoptical transmitter and a pass band wavelength of the Mach-Zehnderinterferometer, through synchronous detection of the secondlow-frequency signal output from the infinitesimal-modulated signalcomponent detection circuit using the first low-frequency signal outputfrom the low-frequency signal generation circuit; a control circuitwhich outputs a control signal to adjust the phase difference betweenthe two split signal light beams so as to correct the shift amount; anda driver circuit which drives the phase adjustment terminal based on thecontrol signal.

In an optical receiver of this invention, the infinitesimal-modulatedsignal component detection circuit may comprise: an eye-openingmonitoring circuit which outputs a signal obtained by monitoring an eyeopening of a signal split from the signal output from the balanceddetection circuit; and a band-pass filter which passes the secondlow-frequency signal contained in the signal output from the eye-openingmonitoring circuit, and the synchronous detection circuit may detect theshift amount and direction based on an output signal of the band-passfilter.

In an optical receiver of this invention, the infinitesimal-modulatedsignal component detection circuit may comprise: a data regenerationcircuit which discriminates and regenerates an electrical signal outputfrom the balanced detection circuit and which is provided with a codeerror detection function; an error count monitoring circuit whichoutputs a signal obtained by monitoring the number of errors output fromthe data regeneration circuit; and a band-pass filter which passes thesecond low-frequency signal contained in the signal output from theerror count monitoring circuit, the synchronous detection circuit maydetects the shift amount and direction based on an output signal of theband-pass filter.

In an optical receiver of this invention, the balanced detection circuitmay comprises an equalizing amplification circuit, theinfinitesimal-modulated signal component detection circuit may comprise:a current consumption monitoring circuit which outputs a signal obtainedby monitoring the current consumption of the equalizing amplificationcircuit; and a band-pass filter which passes the second low-frequencysignal contained in the signal output by the current consumptionmonitoring circuit, and the synchronous detection circuit may detect theshift amount and direction based on an output signal of the band-passfilter.

In an optical receiver of this invention, the balanced detection circuitmay comprise: an optical splitting unit which splits into two each ofthe two output ports of the Mach-Zehnder interferometer; an opticalcoupling unit which causes interference between two light beams split bythe optical splitting unit; and an optical detection unit which convertsan optical signal output from the optical coupling unit into anelectrical signal, the infinitesimal-modulated signal componentdetection circuit may comprise a band-pass filter which passes thesecond low-frequency signal contained in the electrical signal outputfrom the optical detection unit, the synchronous detection circuit maydetect the shift amount and direction based on an output signal of theband-pass filter.

In an optical receiver of this invention, a free spectral range of theMach-Zehnder interferometer may be shifted somewhat from the clock rateof a main signal, the infinitesimal-modulated signal component detectioncircuit may comprise: a first amplifier which amplifies an opticalcurrent of one of photodetectors forming the balanced optical detectioncircuit; and a band-pass filter which extracts a component of the secondlow-frequency signal from an output of the first amplifier, thesynchronous detection circuit may detect the shift amount and directionbased on an output signal of the band-pass filter.

In an optical receiver of this invention, the infinitesimal-modulatedsignal component detection circuit may comprise: a second amplifierwhich amplifies an optical current of the other photodetector formingthe balanced detection circuit; and a subtracter which outputs adifference between the output of the first amplifier and an output ofthe second amplifier, the band-pass filter may extract the component ofthe second low-frequency signal from an output of the subtracter.

In an optical receiver of this invention, the infinitesimal-modulatedsignal component detection circuit may comprise: a clock extractioncircuit which extracts a clock signal from a signal train output fromthe balanced detection circuit; and a low-frequency signal extractioncircuit which extracts the second low-frequency signal superposed on theclock signal output from the clock extraction circuit, the synchronousdetection circuit may detect the shift amount and direction based on thesecond low-frequency signal output from the low-frequency signalextraction circuit.

In an optical transmission receiver of this invention, theinfinitesimal-modulated signal component detection circuit may comprise:a data regeneration circuit which discriminates and regenerates anelectrical signal output from the balanced detection circuit; acorrelation detection circuit which detects a correlation between anoutput signal of the data regeneration circuit and a signal beforediscrimination; and a low-frequency signal extraction circuit whichextracts the second low-frequency signal from an output of thecorrelation detection circuit.

In an optical receiver of this invention, the optical receiver maycomprise: an intensity modulation unit which performs intensitymodulation of the phase-modulated light using a signal at frequency f2sufficiently high to enable superpositioning of the low-frequency signalat frequency f1; and an intensity-modulated component detection unitwhich detects an intensity-modulated component at the frequency f2, theinfinitesimal-modulated signal component detection circuit may extractthe second low-frequency signal at frequency f1 superposed onto thedetected intensity-modulated component at the frequency f2.

In an optical receiver of this invention, as the intensity modulationunit, the optical receiver may comprise: an oscillation circuit whichgenerates a signal at the frequency f2; and an intensity modulator whichperforms intensity modulation of signal light using the signal outputfrom the oscillation circuit.

In an optical receiver of this invention, as the intensity modulationunit, the optical receiver may comprise: an oscillation circuit whichgenerates a signal at the frequency f2; and an optical amplifierconnected to the oscillation circuit, and that the gain of the opticalamplifier may be modulated by the oscillation circuit at the frequencyf2.

In an optical receiver of this invention, as the intensity-modulatedcomponent detection unit, the optical receiver may comprise: an opticalsplitting circuit which splits one of the two output ports of theMach-Zehnder interferometer; a monitoring photodetector connected to theoptical splitting circuit; and an extraction circuit which extracts thecomponent at the frequency f2 from intensity-modulated light output fromthe monitoring photodetector.

In an optical receiver of this invention, as the intensity-modulatedcomponent detection unit, the optical receiver may comprise: an inputlevel adjustment unit which renders asymmetric input levels of theconverted intensity-modulated light which is input to the balanceddetection circuit; and an extraction circuit which extracts thecomponent at the frequency f2 from an output signal of the balanceddetection circuit.

In an optical receiver of this invention, the infinitesimal-modulatedsignal component detection circuit may comprise a data regenerationcircuit which discriminates and regenerates an electrical signal outputfrom the balanced detection circuit, the optical receiver may furthercomprise: a logic inversion circuit which inverts the logic of an outputsignal of the data regeneration circuit and outputs an inverted signal;a selection unit which selectively outputs either the output signal ofthe data regeneration circuit or an output of the logic inversioncircuit according to a prescribed logic specification signal; and apolarity selection unit which inverts the polarity of a feedback errorsignal within the control circuit when the output of the logic inversioncircuit has been selected, an amount of correction of the shift betweenthe center wavelength of the phase-modulated light output from theoptical transmitter and the pass band wavelength of the Mach-Zehnderinterferometer may be reduced to ½ or less of a repetition frequency ofthe Mach-Zehnder interferometer.

In an optical receiver of this invention, the optical receiver mayfurther comprise: a temperature detection circuit which detects thetemperature of a substrate of the Mach-Zehnder interferometer; and aloop open/close switch which turns on and off feedback control to theMach-Zehnder interferometer, when the temperature of the substrate ofthe Mach-Zehnder interferometer is not within an appropriate range, theloop which performs the feedback control may be opened, whereas when thetemperature of the substrate of the Mach-Zehnder interferometer iswithin the appropriate range the loop may be closed to perform thefeedback control.

In an optical receiver of this invention, the control circuit mayfurther comprise: a lock detection circuit which detects a locked stateof a loop to perform feedback control to the Mach-Zehnderinterferometer; and a re-locking circuit which re-locks the loop whenthe locked state indicates that the loop is unlocked, when the lockdetection circuit is detecting that the loop is locked, normal feedbackcontrol may be performed, and when the lock detection circuit is notdetecting that the loop is locked, a driving signal applied to the phaseadjustment terminal of the Mach-Zehnder interferometer may be swept, andif the lock detection circuit once again detects that the loop islocked, switching to a state in which the normal feedback control may beperformed.

In an optical receiver of this invention, the Mach-Zehnderinterferometer may comprise two independent phase adjustment terminals,and an output of the infinitesimal-modulated signal oscillation circuitmay be applied to one of the two phase adjustment terminals, while afeedback error signal within the control circuit may be applied to theother of the two phase adjustment terminals.

In an optical receiver of this invention, the optical receiver maycomprise: an optical carrier frequency detection unit which detects,from received signal light detected by the balanced detection circuit, arelative position between an optical carrier frequency and an opticalfrequency characteristic of the Mach-Zehnder interferometer; and anoffset setting circuit which provides an offset to a feedback errorsignal in the control circuit, a value of the offset of the offsetsetting circuit may be adjusted such that the position of the opticalcarrier frequency matches a peak position or bottom position of theoptical frequency characteristic of the Mach-Zehnder interferometer.

EFFECTS OF THE INVENTION

As explained above, by means of this invention the phase difference inthe signal light of the two arms of a Mach-Zehnder interferometerprovided in an optical receiver of an optical transmission systememploying a DPSK-DD scheme can be modulated at a constant frequency, andby detecting the phase of the frequency component, it is possible to setthe optimum operating point of the Mach-Zehnder interferometer matchingthe optical frequency of the light source on the transmitting side, sothat optimal photo-detection characteristics can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of the opticaltransmission system according to a first embodiment of the presentinvention;

FIG. 2 is a block diagram showing the configuration of the opticaltransmission system according to a second embodiment of the presentinvention;

FIG. 3 is a block diagram showing the configuration of the opticaltransmission system according to a third embodiment of the presentinvention;

FIG. 4 is a block diagram showing the configuration of the opticaltransmission system according to a fourth embodiment of the presentinvention;

FIG. 5 is a block diagram showing the configuration of the opticaltransmission system according to a fifth embodiment of the presentinvention;

FIG. 6 is a block diagram showing the configuration of the opticaltransmission system according to a sixth embodiment of the presentinvention;

FIG. 7 is a block diagram showing the configuration of the opticaltransmission system according to a seventh embodiment of the presentinvention;

FIG. 8 is a block diagram showing the configuration of the opticaltransmission system according to an eighth embodiment of the presentinvention;

FIG. 9 is a block diagram showing the configuration of the opticaltransmission system according to a ninth embodiment of the presentinvention;

FIG. 10A shows the input/output characteristic of a Mach-Zehnderinterferometer;

FIG. 10B shows the input/output characteristic of a Mach-Zehnderinterferometer;

FIG. 10C shows the input/output characteristic of a Mach-Zehnderinterferometer;

FIG. 10D shows the relation between the input/output ports and the twoarms of a Mach-Zehnder interferometer;

FIG. 11 is a block diagram showing the basic configuration of an opticalreceiver in an optical transmission system of the present invention;

FIG. 12 explains the phase shift dependence of the balancedphotodetector output;

FIG. 13 is a block diagram showing the configuration of the opticaltransmission system according to a tenth embodiment of the presentinvention;

FIG. 14 is a block diagram showing the configuration of the opticaltransmission system according to an eleventh embodiment of the presentinvention;

FIG. 15 is a block diagram showing the configuration of the opticaltransmission system according to a twelfth embodiment of the presentinvention;

FIG. 16 is a block diagram showing the configuration of the opticaltransmission system according to a thirteenth embodiment of the presentinvention;

FIG. 17 is a block diagram showing the configuration of the opticaltransmission system according to a fourteenth embodiment of the presentinvention;

FIG. 18 shows an FSR shift of a Mach-Zehnder interferometer;

FIG. 19 shows the relation between the FSR shift of a Mach-Zehnderinterferometer and the infinitesimal-modulated signal componentdetection sensitivity;

FIG. 20 shows the eye-opening penalty due to the FSR shift of theMach-Zehnder interferometer;

FIG. 21 is a block diagram showing the configuration of the opticaltransmission system according to a fifteenth embodiment of the presentinvention;

FIG. 22 is a block diagram showing the configuration of the opticaltransmission system according to a sixteenth embodiment of the presentinvention;

FIG. 23 is a block diagram showing the configuration of the opticaltransmission system according to a seventeenth embodiment of the presentinvention;

FIG. 24 is a block diagram showing the configuration of a controlcircuit with a re-locking function;

FIG. 25 shows the operation of a triangular wave generation circuit of acontrol circuit with a re-locking function;

FIG. 26 is a block diagram showing the configuration of a lock detectioncircuit;

FIG. 27 shows the operation of a lock detection circuit and a triangularwave generation circuit;

FIG. 28 is a block diagram showing the configuration of the opticaltransmission system according to an eighteenth embodiment of the presentinvention;

FIG. 29 is a block diagram showing the configuration of the opticaltransmission system according to a nineteenth embodiment of the presentinvention; and,

FIG. 30 is a block diagram showing the configuration of the opticaltransmission system according to a twentieth embodiment of the presentinvention.

EXPLANATION OF THE REFERENCE SYMBOLS

1 optical transmitter, 2 optical receiver, 100 encoder, 101 lightsource, 102 modulator driving circuit, 103 phase modulator, 105 clocksignal generation circuit, 106 oscillation circuit, 110 modulation statecontrol circuit, 111 control signal communication circuit, 200 MZI(Mach-Zehnder interferometer) for DPSK code demodulation, 201 phaseadjustment terminal, 202 balanced photodetector, 203 amplifier, 204 dataregeneration circuit, 205 clock extraction circuit, 207 controller, 209logic inversion circuit, 210 monitoring photodetector, 211 narrow-bandamplifier, 212 differential circuit, 213 filter, 214 amplifier, 215intensity modulator, 216 oscillation circuit, 217 optical amplifier, 218oscillation circuit, 219 optical attenuator, 220 optical splittingcircuit, 221 balanced detection circuit, 222 infinitesimal-modulatedsignal component detection circuit, 223 synchronous detection circuit,224 infinitesimal-modulated signal oscillation circuit, 225 adder, 226driver, 231 eye-opening monitoring circuit, 232 band pass filer, 241error count monitoring circuit, 251 current consumption monitoringcircuit, 252 transimpedance amplifier, 253 limiting amplifier, 254resistor, 255 amplifier, 261 optical splitting circuit, 262 opticalsplitting circuit, 263 optical coupling circuit, 264 photodetector, 265amplification circuit, 271 resistor, 272 amplification circuit, 273resistor, 274 subtracter, 275 amplification circuit, 281 MZI warm-updetection circuit, 282 loop open/close switch, 284 lock detectioncircuit, 285 control circuit with loop re-locking function, 286 MZItemperature monitor, 287 comparator, 291 phase adjustment terminal, 292infinitesimal-modulation operating point setting circuit, 293 driver,294 MZI offset setting circuit, 295 optical carrier frequency detectioncircuit, 297 control signal communication circuit, 2080 power detectioncircuit, 2841 resistor, 2842 resistor, 2843 resistor, 2844 comparator,2845 comparator, 2846 AND circuit, 2851 triangular wave generationcircuit, 2852 amplifier, 2853 adder, 2854 switch, 2855 comparator, 2856switch, 2857 integration circuit, 2858 resistor, 2859 resistor, C1capacitor.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, embodiments of the present invention are explained in detail,referring to the drawings. However, present invention is not limited tothe embodiments explained below, and for example various constituentcomponents of the embodiments may be combined as appropriate.

Prior to explaining embodiments of the present invention, a principle ofpresent invention is explained referring to FIG. 10A through FIG. 10D,FIG. 11, and FIG. 12.

In resolving the first of the above-described issues, a low-frequencysignal is applied to a phase shifter provided in an interferometer inorder to detect the direction of shift between the signal lightwavelength and the interferometer pass band, and the level or phase ofthis low-frequency signal is detected.

FIG. 10A to FIG. 10C show the input/output characteristics of aMach-Zehnder interferometer (hereafter abbreviated to “MZI”) whichconverts phase-modulated light into intensity-modulated light. FIG. 10Ashows the optical intensities at output ports 1 and 2 plotted againstthe phase differences of the light at arm 1 and arm 2. The upper side inFIG. 10A shows the intensity at output port 2, and the lower side showsthe intensity at output port 1. FIG. 10B and FIG. 10C show thetransmittance from the input port to the output ports 1 and 2respectively as functions of the input light frequency. As shown in FIG.10D, phase-modulated light input from the input port of the MZI 200 issplit and provided to arm 1 and arm 2.

After applying a time slot's delay corresponding to the signal bit rateto arm 2, the light in the two arms is caused to interfere, and theresult is output from an output port. At this time, the opticalintensity output from the output port depends on the delay differencebetween the two arms. For example, when the phase difference is 0, theoptical intensity at output port 1 is maximum, and when the phasedifference is π or is −π, the intensity is minimum. That is, when thephase of the phase-modulated light is 0 continuously for two time slots,the output intensity from output port 1 is minimum, and when the phasesare 0, π or π, 0 continuously for two time slots, the output intensityis maximum.

In other words, this point is the optimum operating point of the MZI. Iffor some reason the delay difference between the two arms is shiftedfrom a phase difference of 0, the minimum value of the optical intensityis increased, and the maximum value is decreased. As a result, there isdegradation of the photo-detection sensitivity of the optical receiver.

Here, the input/output characteristic of the MZI 200 in a frequencydomain has a filter characteristic with repetition frequency equal tothe signal bit rate. Upon performing adjustments such that the opticalintensity is maximum when the phase difference between the two arms is0, the frequency at which the MZI transmittance is greatest matches thecenter frequency of the signal light.

FIG. 11 shows the basic configuration of an optical receiver in anoptical transmission system according to the present invention. In thefigure, the optical receiver has an MZI 200, phase adjustment terminal201, balanced detection circuit 221, infinitesimal-modulated signalcomponent detection circuit 222, synchronous detection circuit 223,infinitesimal-modulated signal oscillation circuit 224 which generates alow-frequency signal at frequency f1, and controller 207 which suppliesa bias voltage to the phase adjustment terminal 201 via an adder 225.The synchronous detection circuit 223 may be a circuit which detects theamplitude and phase information such as a multiplier or mixer, or may bea circuit which detects phase information such as a phase comparator orphase detection circuit.

The output signal of the balanced detection circuit 221 has maximumamplitude at the optimum operating point of the MZI for DPSK codedemodulation (below simply “MZI”) 200, as shown in FIG. 12. Uponapplying a voltage (or current) to the phase adjustment terminal 201provided in one arm of the MZI 200, the phase difference between arm 1and arm 2 changes, and so the minimum and maximum values of the outputlight change.

Upon applying a low-frequency signal with frequency f1 to the phaseadjustment terminal 201 from the infinitesimal-modulated signaloscillation circuit 224, at the optimum operating point (A in thefigure) of the MZI 200, the maximum and minimum values fluctuate attwice the rate of the frequency f1. If for example the phase differencechanges by Δφ1 to result in a shift to B in the figure, then the outputvoltage amplitude declines, and the signal with low frequency f1 issuperposed.

Similarly when the phase difference changes to Δφ2 and the operatingpoint shifts to C, the output voltage amplitude decreases and alow-frequency signal with frequency f1 is superposed; but it is seenthat the phase is inverted compared with point B.

From the output of the balanced detection circuit 221, theinfinitesimal-modulated signal component detection circuit 222 extractsthe low-frequency signal with frequency f1 superposed thereupon, andusing the low-frequency signal output from the infinitesimal-modulatedsignal oscillation circuit 224 which applies the low-frequency signalwith frequency f1 to the phase adjustment terminal 201, the direction ofthe shift in operating point (in terms of the frequency domain,equivalent to the shift between the pass band of the MZI 200 and thecenter frequency of the transmitting-side light source) is detectedthrough synchronous detection by the synchronous detection circuit 223,and the voltage (or current) applied to the phase adjustment terminal201 of the MZI 200 is controlled.

In order to resolve the above-described second issue, a logic inversioncircuit positioned in a stage beyond the discriminator, not shown, inthe infinitesimal-modulated signal component detection circuit 222 isused to invert the logic of the received signal as necessary, in orderto reduce the amount of fluctuation in the MZI pass band to ½ or less ofthat in the prior art.

In FIG. 10A to FIG. 10C, in the initial state with no phase control, ifthe MZI operating point is at position π, then it is necessary to eitheradjust the temperature of the MZI substrate, or to perform phaseadjustment and cause the operating point to shift to a phase differenceof 0. However, if the operating point is set at the point at which thephase difference between the outputs from the two arms of the MZI is π,this adjustment becomes unnecessary. However, in this case the logic ofthe output intensity-modulated signal is inverted. Hence if the logic ofthe signal after data discrimination and regeneration is again inverted,the original signal logic is restored.

First Embodiment

The optical transmission system of a first embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the first embodiment of the present invention appears inFIG. 1. In the figure, the optical transmission system includes anoptical transmitter 1, which outputs differential-encoded,phase-modulated light, and an optical receiver 2, which detects thephase-modulated light transmitted from the optical transmitter 1 andperforms demodulation.

The optical transmitter 1 has an encoder 100 which converts the NRZ codeinput signal into an NRZ-I (Inverted) code signal, a light source 101, amodulator driving circuit 102, and a phase modulator 103 which, for themarks and spaces encoded by the encoder 100, outputs phase-modulatedlight with the phase deviation Δφ in the range 0≦Δφ≦π.

The optical receiver 2 has a Mach-Zehnder interferometer (MZI for DPSKcode demodulation) 200 which splits the phase-modulated light receivedfrom the optical transmitter 1 into two beams, delays one of the splitsignal light beams by one bit, and causes both signal light beams tointerfere to effect conversion into intensity-modulated light, and whichhas a phase adjustment terminal 201 capable of setting the phasedifference between the interfering signals, as well as a balanceddetection circuit 221 which performs photoelectric conversion of thesignal light from the two output ports of the Mach-Zehnderinterferometer 200 and outputs the difference between the convertedelectrical signals.

The optical receiver 2 also has an infinitesimal-modulated signalcomponent detection circuit 222, synchronous detection circuit 223,controller 207, infinitesimal-modulated signal oscillation circuit 224,adder 225, and driver 226.

The infinitesimal-modulated signal component detection circuit 222 usesthe signal output from the balanced detection circuit 221 to detect theinfinitesimal-modulated signal (frequency f1) component applied to thephase adjustment terminal 201 of the Mach-Zehnder interferometer 200,and outputs this component to the synchronous detection circuit 223, andin addition discriminates and regenerates the data output from thebalanced detection circuit 221, and outputs the discriminated andregenerated data as the output signal of the optical receiver 2.

The synchronous detection circuit 223, through synchronous detection ofthe infinitesimal-modulated signal detected by theinfinitesimal-modulated signal component detection circuit 222 and theinfinitesimal-modulated signal directly input from theinfinitesimal-modulated signal oscillation circuit 224, detects theamplitude and phase of the infinitesimal-modulated signal componentsuperposed on the optical signal passed by the Mach-Zehnderinterferometer 200. The amplitude and phase here detected are an errorsignal component arising from a shift between the optical signal carrierfrequency and the optical frequency characteristic of the Mach-Zehnderinterferometer; a signal with this amplitude and phase is supplied tothe controller 207 (in general, loop filter and PID control).

Based on the signal supplied by the synchronous detection circuit 223,the controller 207 outputs to the adder 225, as a bias signal, a controlsignal to adjust the phase difference in the two split signal lightbeams, so as to correct the above shift. The adder 225 adds theinfinitesimal-modulated signal output from the infinitesimal-modulatedsignal oscillation circuit 224 to the bias signal, and outputs theaddition signal to the driver 226. The driver 226 drives the phaseadjustment terminal 201 of the Mach-Zehnder interferometer 200 based onthis addition signal. A feedback loop acts so as to cause this errorsignal component to become zero, so that ultimately the optical signalcarrier frequency matches a peak or bottom of the optical frequencycharacteristic of the Mach-Zehnder interferometer 200.

Second Embodiment

The optical transmission system of a second embodiment of the presentinvention is explained. FIG. 2 shows the configuration of the opticaltransmission system of the second embodiment of the present invention.Differences between the optical transmission system of this embodimentand the optical transmission system of the first embodiment are the factthat the balanced detection circuit 221 includes a balancedphotodetector 202 and an amplifier 203, and that a circuit consisting ofa data regeneration circuit 204, clock extraction circuit 205, and powerdetection circuit 2080 is given as a specific example of a circuitconstituting the infinitesimal-modulated signal component detectioncircuit 222. Otherwise the configuration is the same as that of theoptical transmission system of the first embodiment, and the samesymbols are assigned to the same components. In FIG. 2, the driver 226shown in FIG. 1 has been omitted.

Thus the optical receiver 2 has a balanced photodetector 202; amplifier203 which amplifies the signal output from the balanced photodetector202; data regeneration circuit 204 which discriminates and regeneratesdata from the output of the amplifier 203; clock extraction circuit 205which extracts the clock from the signal train output from the balancedphotodetector 202 via the amplifier 203; infinitesimal-modulated signaloscillation circuit 224 which applies a low-frequency signal withfrequency f1 to the phase adjustment terminal 201 of the MZI 200; powerdetection circuit 2080 which, by detecting the power of the clock signaloutput from the clock extraction circuit 205, extracts the low-frequencysignal with frequency f1 superposed on the clock signal; synchronousdetection circuit 223 which compares the phases of the low-frequencysignal with frequency f1 output from the power detection circuit 2080and the low-frequency signal with frequency f1 output from theinfinitesimal-modulated signal oscillation circuit 224, and detects theamount and direction of the shift between the center wavelength of thephase-modulated light output from the optical transmitter 1 and the passband wavelength of the MSI 200; controller 207 which outputs to theadder 225 a control signal to adjust the phase difference between thetwo split signal light beams, so as to correct the shift amount; andadder 225 which adds the outputs of the infinitesimal-modulated signaloscillation circuit 224 and controller 207 and applies the result to thephase adjustment terminal 201.

The clock extraction circuit 205 must perform linear extraction so as toresult in clock power proportional to the power of the clock componentcontained in the signal.

Third Embodiment

The optical transmission system of a third embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the third embodiment of the present invention is shown in FIG.3. In terms of configuration, the optical transmission system of thisembodiment differs from the optical transmission system of the secondembodiment in that a logic inversion circuit 209 which performs logicinversion of signals according to a logic specification signal inputfrom outside is added to a stage beyond the data regeneration circuit204 of the optical receiver 2, and in that the logic specificationsignal is supplied to the controller 207; otherwise the configuration isthe same as that of the optical transmission system of the secondembodiment shown in FIG. 2, and so the same symbols are assigned to thesame components and redundant explanations are omitted. Also, theoptical transmitter 1 is omitted from the figure.

In FIG. 3, the data regeneration circuit 204 of the optical receiver 2discriminates and regenerates the signal train output from the balancedphotodetector 202, and the logic inversion circuit 209 inverts the logicof the output signal of the data regeneration circuit 204 based on thelogic specification signal input from outside, that is, performsinversion as necessary, and outputs the result.

The logic specification signal input from outside functions to cause thelogic inversion circuit 209 to selectively output either the outputsignal of the data regeneration circuit 204, or the signal resultingfrom logic inversion by the logic inversion circuit 209; the functionalportion for output of this signal may be provided within the opticalreceiver. This logic specification signal, or the functional portionwhich generates this logic specification signal, are equivalent to theselection unit in this invention.

By having the logic inversion circuit 209 perform logic inversion asnecessary, the amount of correction of the shift between the centerwavelength of the phase-modulated light output from the opticaltransmitter 1 and the pass band wavelength of the Mach-Zehnderinterferometer 200 can be made ½ or less than the repetition frequencyof the MZI 200, even when the pass band of the MZI 200 is maximum orminimum at the center wavelength of the phase-modulated light outputfrom the optical transmitter 1.

The logic inversion circuit 209 can easily be configured as an EXOR(Exclusive OR) circuit. The logic specification signal is input fromoutside, but a method may also be used in which the logic specificationsignal is generated by detecting frame information in the output signalof the optical receiver 2 and judging the logic to be specifiedautomatically; or, a command or similar may be input manually.

As explained above, the logic specification signal is also input to thecontroller 207, and when logic inversion is necessary the polarity ofthe bias voltage applied to the phase adjustment terminal 201 must beinverted (or, the direction of the bias current being passed must beinverted).

In FIG. 3, a case of application to the second embodiment is shown; butapplication to other embodiments is also possible.

Fourth Embodiment

The optical transmission system of a fourth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the fourth embodiment of the present invention is shown inFIG. 4. In terms of configuration, the optical transmission system ofthis embodiment differs from the optical transmission system of thesecond embodiment in that the optical transmitter 1 is provided with aclock signal generation circuit 105 which generates a clock signal withthe signal bit rate and an intensity modulator 104 which performsintensity modulation using the clock signal output by the clock signalgeneration circuit 105; in that, in place of the clock extractioncircuit, the optical receiver 2 is provided with an optical splittingcircuit 220 which splits one of the ports among the two output ports ofthe MZI 200, a monitoring photodetector 210 connected to the opticalsplitting circuit 220, and a narrow-band amplifier 211 which extractsthe clock with low-frequency signal at frequency f1 superposed from theintensity-modulated light output from the monitoring photodetector 210;and in that, based on the output signal of the narrow-band amplifier211, the power detection circuit 2080 extracts the low-frequency signalat frequency f1 superposed on the clock, and based on the output of thepower detection circuit 2080, the synchronous detection circuit 223detects the shift amount and direction between the center wavelength ofthe phase-modulated light output from the optical transmitter 1 and thepass band wavelength of the MZI 200. Otherwise, the configuration is thesame as that of the optical transmission system of the second embodimentshown in FIG. 2, and so the same symbols are assigned to the samecomponents, and redundant explanations are omitted.

By having the intensity modulator 104 provided in the opticaltransmitter 1 perform intensity modulation using the clock signal outputfrom the clock signal generation circuit 105, an RZ-DPSK signal isgenerated.

Through intensity modulation of the phase-modulated light on the side ofthe optical transmitter 1, the clock extraction circuit in the opticalreceiver 2 can be simplified. The modulation code of optical signalsgenerated on the side of the optical transmitter 1 may also beCSRZ-DPSK.

Fifth Embodiment

The optical transmission system of a fifth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the fifth embodiment of the present invention is shown in FIG.5. In terms of configuration, the optical transmission system of thisembodiment differs from the optical transmission system of the secondembodiment in that, in the optical receiver 2, a differential circuit212 is provided in place of the clock extraction circuit 205 to detectthe correlation, that is, the difference between, the output signal ofthe data regeneration circuit 204 and the signal before datadiscrimination; otherwise, the configuration is the same as that of theoptical transmission system of the second embodiment shown in FIG. 2,and so the same symbols are assigned to the same components, andredundant explanations are omitted. The differential circuit 212 isequivalent to the correlation detection circuit of the presentinvention. The optical transmitter 1 is omitted from the figure.

Instead of synchronously detecting and extracting the low-frequencysignal at frequency f1 superposed on the clock signal output from theclock extraction circuit 205 in the second embodiment, the differentialcircuit 212 determines the correlation between the data signal beforediscrimination and regeneration by the data regeneration circuit 204 andthe data signal after discrimination and regeneration, and the powerdetection circuit 2080 extracts the low-frequency signal at frequency f1from the output of the differential circuit 212; based on the output ofthe power detection circuit 2080, the synchronous detection circuit 223detects the amount and direction of the shift between the centerwavelength of the phase-modulated light from the optical transmitter 1and the pass band wavelength of the MZI 200.

The data signal before data discrimination and regeneration in the dataregeneration circuit 204 has a low frequency f1 superposed, but the datasignal after discrimination and regeneration does not have a lowfrequency superposed, and so by detecting this difference in thedifferential circuit 212, it is possible to detect only thelow-frequency component.

Sixth Embodiment

The optical transmission system of a sixth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the sixth embodiment of the present invention is shown in FIG.6. In terms of configuration, the optical transmission system of thisembodiment differs from the optical transmission system of the secondembodiment in that the optical transmitter 1 is provided with anoscillation circuit 106 which generates a signal at a frequency f2sufficiently high to enable superpositioning of the low-frequency signalat frequency f1 which directly intensity-modulates the light source 101,and moreover, in place of the clock extraction circuit 205, the opticalreceiver 2 is provided with an optical splitting circuit 220 whichsplits one of the ports among the two output ports in the MZI 200, amonitoring photodetector 210 connected to the optical splitting circuit220, and an amplifier 214 and a filter 213 which extract the componentat frequency f2 superposed on the low-frequency signal at the frequencyf1 from the intensity-modulated light output from the monitoringphotodetector 210. Otherwise, the configuration is the same as that ofthe optical transmission system of the second embodiment shown in FIG.2, and so the same symbols are assigned to the same components, andredundant explanations are omitted.

The power detection circuit 2080, instead of synchronously detecting andextracting the low-frequency signal at frequency f1 superposed on theclock signal output from the clock extraction circuit 205 in the secondembodiment, extracts the low-frequency signal at frequency f1 superposedon the component at frequency f2 output from the filter 213, and thesynchronous detection circuit 223 detects the amount and direction ofthe shift between the center wavelength of the phase-modulated lightoutput from the optical transmitter 1 and the pass band wavelength ofthe MZI 200, based on the output of the power detection circuit 2080.

The amplifier 214 and filter 213 are equivalent to the signal detectionunit of the present invention.

Here, in the optical transmitter 1 the output of the light source 101 isintensity-modulated at frequency f2 by the output signal of theoscillation circuit 106. At this time, the frequency f2 must besufficiently high to enable superpositioning of the low-frequency signalat frequency f1; and a frequency must be selected in a range above thelow-range cutoff frequency of the optical amplifier positioned in thetransmission path.

The frequency f1 is superposed by the MZI 200 of the optical receiver 2onto the intensity-modulated component at frequency f2 superposed on theoutput signal light of the optical transmitter 1, and the result isoutput.

In the monitoring photodetector 210, optical signals split by one of theports of the MZI 200 are detected, and after amplification by theamplifier 214 the signal at frequency f2 superposed thereupon isdetected by the filter 213.

An advantage of this scheme is that there is no need to use a monitoringphotodetector, later-stage amplifier, power detection circuit,synchronous detection circuit, or other products with excellenthigh-frequency characteristics.

The intensity-modulated component which has been intensity-modulated onthe transmitting side is not output by the balanced photodetector 202,and so there is no large impact on signal regeneration in the dataregeneration circuit 204. However, the levels of the two input signalsto the balanced photodetector 202 must be made to match. In the opticaltransmission system shown in FIG. 6, a monitoring terminal is providedat one port, and so it is necessary to add a loss equivalent to the lossof this terminal to the other port as well.

Seventh Embodiment

The optical transmission system of a seventh embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the seventh embodiment of the present invention is shown inFIG. 7. In terms of configuration, the optical transmission system ofthis embodiment differs from the optical transmission system of thesixth embodiment in that, in place of intensity modulation of the lightsource on the transmitting side, an oscillation circuit 216 whichgenerates a signal at frequency f2 and an intensity modulator 215 whichperforms intensity modulation of the signal light using the outputsignal of the oscillation circuit 216 are provided in the opticalreceiver 2; otherwise, the configuration is the same as that of theoptical transmission system of the sixth embodiment shown in FIG. 6, andso the same symbols are assigned to the same components, and redundantexplanations are omitted. The optical transmitter 1 is omitted from thefigure.

Here, the intensity modulator 215 is positioned in the input stage ofthe optical receiver 2, and performs intensity modulation using a signalat frequency f2 output from the oscillation circuit 216. This intensitymodulator 215 may for example be a LN (Lithium Niobate) modulator, an AO(Acoust-Optic) modulator, or an electroabsorption optical modulator.

Eighth Embodiment

The optical transmission system of an eighth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the eighth embodiment of the present invention is shown inFIG. 8. In terms of configuration, the optical transmission system ofthis embodiment differs from the optical transmission system of theseventh embodiment in that, in place of the intensity modulator 215 inthe optical receiver 2, an optical amplifier 217 is provided, and thegain of the optical amplifier 217 is modulated at frequency f2 by anoscillation circuit 218 which generates a signal at frequency f2;otherwise, the configuration is the same as that of the opticaltransmission system of the seventh embodiment shown in FIG. 7, and sothe same symbols are assigned to the same components, and redundantexplanations are omitted. The optical transmitter 1 is omitted from thefigure.

When a modulator is used, the SN ratio degradation due to the insertionloss poses a problem, and so in this embodiment the gain of an opticalamplifier 217 is modulated. In particular, when using an opticalamplifier, the ability to perform modulation using only a receiveramplifier is advantageous when considering application to WDM systems.

In both the seventh and the eighth embodiments, even if intensitymodulation is performed an intensity-modulated component is not outputby the balanced photodetector 202, and so there is no large impact onsignal regeneration. However, the levels of the two input signals to thebalanced photodetector 202 must be made to match. In the figures, amonitoring terminal is provided at one port, and so it is necessary toadd a loss equivalent to the loss of this terminal to the other port aswell.

Ninth Embodiment

The optical transmission system of a ninth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the ninth embodiment of the present invention is shown in FIG.9. In terms of configuration, the optical transmission system of thisembodiment differs from the optical transmission system of the eighthembodiment in that, in place of provision of the optical splittingcircuit 220 and the monitoring photodetector 210, the optical receiver 2is provided with input level adjustment unit, which renders asymmetricalthe input levels of converted intensity-modulated light for input to thebalanced photodetector 202; otherwise, the configuration is the same asthat of the optical transmission system of the eighth embodiment shownin FIG. 8, and so the same symbols are assigned to the same components,and redundant explanations are omitted. The optical transmitter 1 isomitted from the figure.

On the side of the optical transmitter 1, or on the side of the opticalreceiver 2, signal light intensity-modulated at the frequency f2 is notoutput if the averaged signal power input to the two input ports of thebalanced photodetector 202 is the same. In other words, if the averagedinput power to one of the input ports is intentionally lowered, anintensity-modulated component can be detected.

In this embodiment, an optical attenuator 219 is connected to one of theinput ports for this reason. The optical attenuator 219 is equivalent tothe input level adjustment unit of the present invention.

The intensity-modulated component at frequency f2 detected by thebalanced photodetector 202 is input to the power detection circuit 2080via the filter 213, and is used in control of the MZI 200. A capacitorC1 is connected to the input terminal of the data regeneration circuit204, and by blocking this intensity-modulated component, there is nolarge impact on the signal discrimination and regeneration in the dataregeneration circuit 204.

In the above-described sixth through ninth embodiments, three differentmethods of intensity modulation at frequency f2 (direct intensitymodulation of the light source in the optical transmitter 1, intensitymodulation in the optical receiver 1 using an intensity modulator, andintensity modulation in the optical receiver 1 using an opticalamplifier), as well as two methods of detection of the frequency f2(monitoring of one of the ports of the MZI 200, connection of an opticalattenuator 219 to one of the input ports of the balanced photodetector202); however, configurations are not limited to those explained in thesixth through ninth embodiments, and the these methods may be freelycombined.

Tenth Embodiment

The optical transmission system of a tenth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the tenth embodiment of the present invention is shown in FIG.13. In terms of configuration, the optical transmission system of thisembodiment differs from the optical transmission system of the firstembodiment in that, similarly to for example FIG. 2, the balanceddetection circuit 221 consists of a balanced photodetector 202 and anamplifier 203, and in that the infinitesimal-modulated signal componentdetection circuit 222 consists of a data regeneration circuit 204 whichdiscriminates and regenerates data from the output of the amplifier 203,an eye-opening monitoring circuit 231 which monitors the opening of theeye pattern of the main signal output from the balanced detectioncircuit 221, and a band-pass filter 232 which passes theinfinitesimal-modulated signal component (f1). Otherwise, theconfiguration is the same as that of the optical transmission systemshown in FIG. 1, and so the same symbols are assigned to the samecomponents, and redundant explanations are omitted.

When the signal light carrier frequency shifts from the peak or bottomof the optical frequency characteristic of the Mach-Zehnderinterferometer 200, the amplitude of the main signal detected by thebalanced photodetector 202 and output from the amplifier 203 is reduced,or, the S/N ratio is degraded. Hence by monitoring the eye opening ofthe main signal in the eye-opening monitoring circuit 231, and byextracting the infinitesimal-modulated signal component (f1) using theband-pass filter 232, the amplitude and phase of theinfinitesimal-modulated signal component superposed on the opticalsignal passed by the Mach-Zehnder interferometer 200 can be detected.Through synchronous detection of this signal by the synchronousdetection circuit 223 an error signal can be extracted, and this errorsignal can be fed back to enable locking on the desired state.

The most important advantage of this embodiment is the ability to alwaysstabilize operation at the point at which the eye opening is maximum.

Eleventh Embodiment

The optical transmission system of an eleventh embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the eleventh embodiment of the present invention is shown inFIG. 14. In terms of configuration, the optical transmission system ofthis embodiment differs from the optical transmission system of thefirst embodiment in that the balanced detection circuit 221 consists ofa balanced photodetector 202 and an amplifier 203, and in that theinfinitesimal-modulated signal component detection circuit 222 consistsof a data regeneration circuit 204, which discriminates and regeneratesdata from the output of the amplifier 203 and also provides code errordetection functions, an error count monitoring circuit 241 whichmonitors the number of errors, and a band-pass filter 232 as shown inFIG. 13. Otherwise, the configuration is the same as that of the opticaltransmission system shown in FIG. 1, and so the same symbols areassigned to the same components, and redundant explanations are omitted.

When the signal light carrier frequency shifts from the peak or bottomof the optical frequency characteristic of the Mach-Zehnderinterferometer, code errors occur in the data regenerated by the dataregeneration circuit 204. Hence by using the error count monitoringcircuit 241 to monitor the number of code errors, and by extracting theinfinitesimal-modulated signal component (f1) using the band-pass filter232, the amplitude and phase of the infinitesimal-modulated signalcomponent superposed on the optical signal passed by the Mach-Zehnderinterferometer 200 can be detected. By synchronously detecting thissignal using the synchronous detection circuit 232, the error signalcomponent can be extracted, and by feeding back this error signalcomponent, it is possible to lock on the desired state.

In the above explanation, the number of code errors was used; in placeof this, the number of code errors corrected may be used.

The most important advantage of this embodiment is the ability to alwaysstabilize operation at the point at which the code error rate isminimum.

Twelfth Embodiment

The optical transmission system of a twelfth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the twelfth embodiment of the present invention is shown inFIG. 15. The optical transmission system of this embodiment differs fromthe optical transmission system of the first embodiment in that thebalanced detection circuit 221 consists of an equalizing amplificationcircuit, which is equivalent to the balanced photodetector 202 andamplifier 203, and in that the infinitesimal-modulated signal componentdetection circuit 222 consists of a data regeneration circuit 204, whichdiscriminates and regenerates data from the output of the equalizingamplification circuit, a current consumption monitoring circuit 251,which monitors the current consumption of the equalizing amplificationcircuit constituting the balanced detection circuit 221, and theband-pass filter 232 shown in FIG. 13.

An equalizing amplification circuit generally consists of atransimpedance amplifier (TIA) 252 and a limiting amplifier (LIM) 253.The current consumption monitoring circuit 251 consists of a resistor254 inserted between the power supply terminal of the limiting amplifier253 and the power supply, and an amplifier 255 which amplifies andoutputs the voltage at this power supply terminal. Otherwise, theconfiguration is the same as that of the optical transmission systemshown in FIG. 13, and so the same symbols are assigned to the samecomponents, and redundant explanations are omitted.

When the carrier frequency of the signal light shifts from the peak orbottom of the optical frequency characteristic of the Mach-Zehnderinterferometer 200, the amplitude of the main signal input to theequalizing amplification circuit is reduced. The transistoramplification circuit constituting the equalizing amplification circuitis generally such that the current value flowing in the transistor isasymmetric when the input signal voltage (current) deviates in thepositive direction and in the negative direction; hence the currentconsumption differs depending on the amplitude of the input signal tothe transistor amplification circuit.

Hence the current consumption monitoring circuit 251 is used to monitorcurrent consumption in the equalizing amplification circuit, and byextracting the infinitesimal-modulated signal component (f1) using theband-pass filter 232, the amplitude and phase of theinfinitesimal-modulated signal component superposed on the opticalsignal passed by the Mach-Zehnder interferometer 200 can be detected. Bysynchronous detection of this signal using the synchronous detectioncircuit 223 the error signal component can be extracted, and by feedingback this error signal component, it is possible to lock on the desiredstate.

The most important advantage of this embodiment is the ability to detectthe main signal peak without having to use main signal splitting, whichhas a large impact on the main signal.

Thirteenth Embodiment

The optical transmission system of a thirteenth embodiment of thepresent invention is explained. The configuration of the opticaltransmission system of the thirteenth embodiment of the presentinvention is shown in FIG. 16. The optical transmission system of thisembodiment differs from the optical transmission system of the firstembodiment in the following respects.

The balanced detection circuit 221 consists of optical splittingcircuits 261 and 262 provided on each of the two output arms of theMach-Zehnder interferometer 200; an optical coupling circuit 263 whichcouples the two optical signals split by the splitting circuits; abalanced photodetector 202; an amplifier 203; a photodetector 264 whichdetects light coupled by the optical coupling circuit 263; and anamplification circuit 265 which amplifies the electrical signals outputfrom the photodetector 264.

Also, the infinitesimal-modulated signal component detection circuit 222consists of a data regeneration circuit 204, which discriminates andregenerates data from the output of the amplifier 203, and a band-passfilter 232 which passes the infinitesimal-modulated signal component(f1) output from the amplification circuit 265. Here, the light in thetwo optical paths which are split and then recoupled have equal bitlengths but are opposite in optical phase. Otherwise, the configurationis the same as that of the optical transmission system shown in FIG. 1,and so the same symbols are assigned to the same components, andredundant explanations are omitted.

When the carrier frequency of the signal light shifts from the peak orbottom of the optical frequency characteristic of the Mach-Zehnderinterferometer 200, light output from the two output ports of theMach-Zehnder interferometer 200 has the peak power reduced at marks andincreased at spaces. By causing these two optical signals to interferewith equal lengths and opposite phases using the optical splittingcircuits 261 and 262 as well as the optical coupling circuit 263, whenthe signal light carrier frequency shifts from the peak or bottom of theoptical frequency characteristic of the Mach-Zehnder interferometer 200,light with peak power reduced on the mark side is made to interfere,with opposite phases, with light with peak power increased on the spaceside, so that the peak power and averaged power of the light afterinterference can be reduced. This power fluctuation is detected by aphotodetector 264, and by extracting the infinitesimal-modulated signalcomponent (f1) using the band-pass filter 232 via the amplificationcircuit 265, the amplitude and phase of the infinitesimal-modulatedsignal component superposed on the optical signal passed by theMach-Zehnder interferometer 200 can be detected. By synchronouslydetecting this signal using the synchronous detection circuit 223 theerror signal component can be extracted, and by feeding back this errorsignal component, it is possible to lock on the desired state.

The most important advantage of this embodiment is the ability to detectthe infinitesimal-modulated signal component using a comparatively slow(f1) photodetector, without performing main signal splitting in theelectrical domain.

Fourteenth Embodiment

The optical transmission system of a fourteenth embodiment of thepresent invention is explained. The configuration of the opticaltransmission system of the fourteenth embodiment of the presentinvention is shown in FIG. 17. The optical transmission system of thisembodiment differs from the optical transmission system of the firstembodiment in that the balanced detection circuit 221 is configured froma balanced photodetector 202, amplifier 203, and resistor 271 to apply abias voltage to the balanced photodetector 202 from a positive (+) powersupply; in that the infinitesimal-modulated signal component detectioncircuit 222 is configured from a data regeneration circuit 204 whichdiscriminates and regenerates data from the output of the amplifier 203,an amplification circuit 272 which detects the optical current flowingin one of the photodetectors of the balanced photodetector 202 andamplifies the current, and the band-pass filter 232 shown in FIG. 13;and in that the FSR (free spectral range) of the Mach-Zehnderinterferometer 200 is set to be somewhat larger than the main signalclock rate (that is, as described below, a prescribed amount within therange in which the main signal penalty can be ignored).

In for example FIG. 13 through FIG. 16, a resistor 271 is in actualityprovided, but because the operation in these figures was not directlyrelated, the resistor was not shown. Otherwise the configuration is thesame as that of the optical transmission system shown in FIG. 1, and sothe same symbols are assigned to the same components, and redundantexplanations are omitted.

In a system using a DPSK-DD scheme, the optical signal modulation bandis broad compared with the FSR of the Mach-Zehnder interferometer, andthere is almost no change in the optical power even if the opticalcarrier frequency shifts from the peak or bottom of the opticalfrequency characteristic of the Mach-Zehnder interferometer. Hencedetection of an infinitesimal-modulated signal component superposed onthe optical signal is difficult. In the case of an RZ-type DPSK signalin particular, the modulation spectrum is broad compared with NRZ-typeDPSK signals, and so detection is still more difficult. In thisembodiment, the FSR of the Mach-Zehnder interferometer 200 is madesufficiently large that there is no penalty imparted to the main signalclock rate, and consequently, in relative terms the optical signalmodulation band is equivalently narrowed, and theinfinitesimal-modulated signal component can be easily detected.

FIG. 18 shows the FSR shift of the Mach-Zehnder interferometer 200. TheMZI transmission characteristic 1 is the optical frequencycharacteristic of a Mach-Zehnder interferometer having an FSR (labeled“FSR1 (reference)” in the figure) equal to the main signal clock rate;the MZI transmission characteristic 2 is the optical frequencycharacteristic of a Mach-Zehnder interferometer having an FSR (“FSR2(shifted)” in the figure) somewhat larger than the main signal clockrate. The FSR shift is found by taking the difference FSR2−FSR1.

FIG. 19 shows the relation between the FSR shift and theinfinitesimal-modulated signal component detection sensitivity(variation in averaged optical power/averaged optical power). As the FSRshift is increased, the variation in the averaged optical power is seento increase.

FIG. 20 shows the main signal eye-opening penalty due to the FSR shift.Within approximately 10% of the bit rate, it is seen that theeye-opening penalty can be held to 0.1 dB or less. Hence by setting theFSR to be somewhat larger than the main signal clock rate, the mainsignal averaged optical power can be detected with almost no penaltyimparted to the main signal, through synchronous detection of the signalby the synchronous detection circuit 223 the error signal component canbe extracted, and the error signal component can be fed back to enablelocking on the desired state.

The most important advantage of this embodiment is the ability toperform control using a comparatively slow (f1) signal obtained from thepower supply terminal of the balanced photodetector 202, withoutsplitting the main signal in the electrical domain.

Fifteenth Embodiment

The optical transmission system of a fifteenth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the fifteenth embodiment of the present invention is shown inFIG. 21. The optical transmission system of this embodiment differs fromthe optical transmission system of the fourteenth embodiment in that,rather than taking a signal from the power supply terminal of thephotodetector on only one side (the positive power supply side) of thebalanced photodetector 202, by using an amplification circuit 275 todetect the optical current of the photodetector on the other side of thebalanced photodetector 202 connected to the negative power supply via aresistor 273, signals can be taken from the power supply terminals ofthe photodetectors on both sides, the difference between the two signalscan be determined using a subtracter 274, and this difference can beused in feedback control similar to that in the fourteenth embodiment.Otherwise the configuration is the same as that of the opticaltransmission system shown in FIG. 17, and so the same symbols areassigned to the same components, and redundant explanations are omitted.By performing balanced detection of the feedback signal for controlsimilarly to the main signal, the detection sensitivity can be furtherraised.

The most important advantages of this embodiment are the ability toperform control using a comparatively slow (f1) signal obtained from thepower supply terminals of the balanced photodetector 202, withoutsplitting the main signal in the electrical domain, and thecomparatively high detection sensitivity.

Sixteenth Embodiment

The optical transmission system of a sixteenth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the sixteenth embodiment of the present invention is shown inFIG. 22. The optical transmission system of this embodiment differs fromthe optical transmission system of the first embodiment in that a logicinversion circuit 209 is connected in a stage beyond theinfinitesimal-modulated signal component detection circuit 222. Thisembodiment illustrates the application of the technical concept of thethird embodiment to the first embodiment; because the logic inversioncircuit 209 is the same as that shown in FIG. 3, the same symbol isassigned, and a redundant explanation is omitted.

The most important advantage of this embodiment is the ability to reducethe maximum value of the initial setting applied to the phase adjustmentterminal of the Mach-Zehnder interferometer to ½ or less of the valuewhen a logic inversion circuit is not used.

Seventeenth Embodiment

The optical transmission system of a seventeenth embodiment of thepresent invention is explained. The configuration of the opticaltransmission system of the seventeenth embodiment of the presentinvention is shown in FIG. 23. The optical transmission system of thisembodiment differs from the optical transmission system of the firstembodiment in that the optical receiver 2 has an MZI warm-up detectioncircuit 281, which detects the warmed-up state based on the temperatureof the Mach-Zehnder interferometer 200; a loop open/close switch 282,which, by opening and closing the control loop which performs feedbackcontrol to the Mach-Zehnder interferometer 200, can turn on/off feedbackcontrol to the Mach-Zehnder interferometer 200; a synchronous detectioncircuit 223, which compares the phase of the infinitesimal-modulatedsignal detected by the infinitesimal-modulated signal componentdetection circuit 222 with the phase of the infinitesimal-modulatedsignal output from the infinitesimal-modulated signal oscillationcircuit 224, and outputs an error signal; a lock detection circuit 284,which, upon detecting the locked state of the control loop based on theerror signal from the synchronous detection circuit 223, outputs a lockdetection signal; and a control circuit with loop re-locking function285, having a function for re-locking when the control loop deviatesfrom a lock on the optical frequency.

Also, the MZI warm-up detection circuit 281 has an MZI temperaturemonitor 286 which monitors the temperature of the substrate of theMach-Zehnder interferometer 200 and outputs a voltage corresponding tothe temperature, and a comparator 287 which compares the output voltagefrom the MZI temperature monitor 286 and a reference voltage Vref1, andoutputs to the loop open/close switch 282 a signal indicating the resultof comparison indicating whether the substrate temperature is within anappropriate range.

In order to use the Mach-Zehnder interferometer at an overall constanttemperature, at the time of system startup some time is required untilthe temperature reaches a preset value. At this time the opticalfrequency characteristic of the Mach-Zehnder interferometer changesrapidly (drifts), and if an attempt were made to initiate control, therewould be the danger of runaway operation. Hence in this embodiment, thecontrol loop is closed only after warm-up of the Mach-Zehnderinterferometer 200 has ended. As a result, an unwanted cause ofinstability can be eliminated.

Further, even if there is a deviation from the lock on the opticalfrequency due to some unforeseen disturbance, by using the lockdetection circuit 284 and the control circuit with loop re-lockingfunction 285, it is possible to again return to the locked state afterthe disturbance has subsided.

FIG. 24 shows an example of the configuration of a control circuit withloop re-locking function 285. The control circuit with loop re-lockingfunction 285 has a triangular wave generation circuit 2851, whichoperates based on a lock detection signal and error signal, an amplifier2852 which amplifies the error signal, and an adder 2853 which adds theoutput of the amplifier 2852 and the output of the triangular wavegeneration circuit 2851 and outputs the result.

The triangular wave generation circuit 2851 has a switch 2854, whichswitches between the reference voltage Vref and ground according to thelock detection signal; a comparator 2855, which compares the signal Bwith the signal C, described below, and outputs the comparison result asan output signal A and the inverted output signal of same; a switch2856, which is a switch operating in concert with the switch 2854, andwhich switches between the error signal and the output signal A of thecomparator 2855 according to the lock detection signal; an integrationcircuit 2857, which integrates the difference of the two signalssupplied via the switch 2854 and the switch 2856; and resistors 2858 and2859, which divide the inverted output signal of the comparator 2855 togenerate the signal C.

When a lock detection signal is detected, one of the inputs to theintegration circuit 2857 is connected to the reference voltage Vref bythe switch 2854, and the other input to the integration circuit 2857 isconnected to the error signal by the switch 2856; by this means the loopof the integration circuit 2857 is closed, and the normal feedbackcontrol state is entered. As a result, the error signal shift from thereference voltage Vref is integrated such that the error signal becomesthe reference voltage Vref. When a lock detection signal is notdetected, one of the inputs to the integration circuit 2857 is connectedto ground by the switch 2854, and the other input to the integrationcircuit 2857 is connected to the output of the comparator 2855 (outputsignal A) by the switch 2856, so that the integration loop is opened,and a triangular wave is generated by the comparator 2855 and theintegration circuit 2857.

FIG. 25 shows the operation of the triangular wave generation circuit2851. As explained above, the output signal A is the output of thecomparator 2855, the signal C is the result of inversion of the outputsignal A, fed back to the input of the comparator 2855, and the signal Bis the triangular wave output from the integrator 2857. In thecomparator 2855, operation is repeated in which signal B and signal Care compared and the case of signal B being greater than signal C isdetected and the output signal A and signal C are inverted, so that thetriangular wave signal B can be output.

FIG. 26 shows an example of the configuration of the lock detectioncircuit 284. The lock detection circuit 284 shown has resistors 2841 to2843, which divide the voltage across the positive and negative powersources to output voltage VH and VL equivalent to the upper limit andthe lower limit respectively of the appropriate range for the substratetemperature, described above; a comparator 2844, which compares thevoltage Vpc input to the lock detection circuit 284 and the voltage VH;a comparator 2845, which compares the voltage Vpc with the voltage VL;and an AND circuit 2846 which takes the logical product of the outputsof the comparators 2844 and 2845. This lock detection circuit is athreshold circuit which judges the loop to be locked when the voltageVpc, which is the voltage of the error signal from the synchronousdetection circuit 223 shown in FIG. 23, is between the voltage VH andthe voltage VL.

FIG. 27 shows the operation of the lock detection circuit 284 andtriangular wave generation circuit 2851. When the lock detection circuit284 judges that the loop is no longer locked, the operation of thecontrol circuit with loop re-locking function 285 switches to theoperation of the triangular wave generation circuit, and the currentapplied to the phase adjustment terminal 201 (see FIG. 23) is sweptusing the triangular wave, as shown in the figure. While sweeping, ifthe output voltage of the synchronous detection circuit 223 enters intothe lock judgment region between the voltage VH and voltage VL, theoperation of the control circuit with loop re-locking function 285switches from triangular wave generation circuit operation tointegration circuit operation, and the control loop is closed.

The most important advantage of this embodiment is that, while renderingunlikely an unstable state, if the loop is no longer locked, the controlcircuit can again effect return to the locked state. In FIG. 23, thecase of application to the first embodiment is shown, but application toother embodiments is also possible.

Eighteenth Embodiment

The optical transmission system of an eighteenth embodiment of thepresent invention is explained. The configuration of the opticaltransmission system of the eighteenth embodiment of the presentinvention is shown in FIG. 28. The optical transmission system of thisembodiment differs from the optical transmission system of the firstembodiment in that the Mach-Zehnder interferometer 200 has two phaseadjustment terminals on each of its two arms (that is, in addition tothe phase adjustment terminal 201 described above, a phase adjustmentterminal 291 is present), and in that an infinitesimal-modulated signalis applied to one of these (in the figure, the phase adjustment terminal291), while a feedback control signal (feedback error signal) is appliedto the other (in the figure, the phase adjustment terminal 201).

Specifically, an adder 225 is not provided, and there are provided aninfinitesimal-modulation operating point setting circuit 292 whichcompares the reference voltage Vref2 and the output of theinfinitesimal-modulated signal oscillation circuit 224 and outputs asignal to set an infinitesimal-modulated signal operating point, adriver 293 which drives the phase adjustment terminal 291 based on theoutput of the infinitesimal-modulation operating point setting circuit292, and an MZI offset setting circuit 294 which compares the referencevoltage Vref3 and the output of the synchronous detection circuit 223and outputs a signal to determine the feedback control signal operatingpoint. Otherwise the configuration is the same as that of the opticaltransmission system shown in FIG. 1, and so the same symbols areassigned to the same components, and redundant explanations are omitted.

For example, in a Mach-Zehnder interferometer which uses a thermoopticeffect as the arm optical phase adjustment unit, the efficiency of phaseadjustment differs depending on the operating point of the drivercircuit. When the infinitesimal-modulated signal and feedback controlsignal are added by the adder 225 and supplied to the same phaseadjustment terminal, the infinitesimal-modulation efficiency changesdepending on the magnitude of the feedback control signal, and so itbecomes extremely difficult to realize stable operation of the loop andto estimate the penalty. In this embodiment, by providing the phaseadjustment terminals for infinitesimal-modulation and for feedbackcontrol, the above problem can be resolved. In addition, by providing aninfinitesimal-modulation operating point setting circuit 292 and MZIoffset setting circuit 294 as reference voltage setting circuits inorder to determine the operating points of the infinitesimal-modulatedsignal and feedback control signal, the respective operating points canbe adjusted independently.

In this embodiment, phase adjustment terminals 201 and 291 are providedin each of the two arms of the Mach-Zehnder interferometer 200. However,if the electrodes are divided, an effect similar to the provision of thephase adjustment terminals can be realized, and so a plurality ofelectrodes may be provided at one of the arms, andinfinitesimal-modulated signal and feedback control signals may beapplied to the different electrodes.

The most important advantage of this embodiment is the ability to applythe infinitesimal-modulated signal with reliably stable efficiency. InFIG. 28, a case of application to the first embodiment is shown, butapplication to other embodiments is also possible.

Nineteenth Embodiment

The optical transmission system of a nineteenth embodiment of thepresent invention is explained. The configuration of the opticaltransmission system of the nineteenth embodiment of the presentinvention is shown in FIG. 29. The optical transmission system of thisembodiment differs from the optical transmission system of theeighteenth embodiment in that the optical receiver 2 has an opticalcarrier frequency detection circuit 295, which detects the position ofthe optical carrier frequency relative to the optical frequencycharacteristic of the Mach-Zehnder interferometer 200, using thedetected reception signal light. Otherwise the configuration is the sameas that of the optical transmission system shown in FIG. 28, and so thesame symbols are assigned to the same components, and redundantexplanations are omitted.

When performing control, as in the thirteenth to fifteenth embodiments,to set the peak or bottom of the optical frequency characteristic of theMach-Zehnder interferometer 200 at the position of maximum averagedpower of the optical signal, there may be cases in which, due to theasymmetry of the optical spectrum of the optical modulated signal, thecontrol stability point does not always match the optical carrierfrequency. In this embodiment, the optical carrier position is detectedby the optical carrier frequency detection circuit 295, and an offsetvalue is applied to the MZI offset setting circuit 294 so as to effectstabilization of the peak or bottom of the optical frequencycharacteristic of the Mach-Zehnder interferometer 200 at this point.

The optical carrier frequency detection circuit 295 must determine theposition of the carrier from the modulation signal without a carrier,and so a method is for example conceivable in which the modulationsignal is scanned using a Fabry-Perot resonator to find two minima inthe optical spectrum, and the optical carrier frequency is taken to bethe midpoint between these two frequencies.

The most important advantage of this embodiment is the ability to causethe peak or bottom of the optical frequency characteristic of theMach-Zehnder interferometer to match the optical carrier frequency, evenwhen the optical modulated signal spectrum is asymmetric. In FIG. 29,this embodiment is applied to the eighteenth embodiment, which is basedon the first embodiment; but application to configurations based onother embodiments is also possible.

Twentieth Embodiment

The optical transmission system of a twentieth embodiment of the presentinvention is explained. The configuration of the optical transmissionsystem of the twentieth embodiment of the present invention is shown inFIG. 30. The optical transmission system of this embodiment differs fromthe optical transmission system of the nineteenth embodiment in that theoptical transmitter 1 has a modulation state control circuit 110 whichcan turn modulation of the main signal on and off, and a control signalcommunication circuit 111 which exchanges control signals with theoptical receiver 2 using a control line provided separately from theline for main signals, and in that the optical receiver 2 has a controlsignal communication circuit 297 which exchanges control signals withthe optical transmitter 1 using the control line. Otherwise theconfiguration is the same as that of the optical transmission systemshown in FIG. 29, and so the same symbols are assigned to the samecomponents, and redundant explanations are omitted.

By enabling the optical transmitter 1 to turn off modulation of mainsignals, the unmodulated optical carrier alone can be transmitted to thereceiving side, and at the optical receiver 2 the optical carrier can beused to easily determine the optical carrier frequency. On the receivingside, the optical carrier frequency detection circuit 295 usesinformation relating to this optical carrier frequency received via thecontrol signal communication circuit 297 to provide an offset value tothe MZI offset setting circuit 294, such that the optical carrierfrequency matches the peak or the bottom frequency of the opticalfrequency characteristic of the Mach-Zehnder interferometer 200.

As the specific operation, upon startup of the optical transmissionsystem the optical transmitter 1 turns off modulation of the mainsignal, and transmits only the optical carrier. The optical receiver 2detects the position of the frequency of the optical carrier sent fromthe optical transmitter 1 relative to the optical frequencycharacteristic of the Mach-Zehnder interferometer 200, and adjusts theoffset value of the MZI offset setting circuit 294 such that theposition of the optical carrier frequency matches the peak or bottom ofthe optical frequency characteristic of the Mach-Zehnder interferometer200. Next, the optical receiver 2 sends a control signal indicatingcompletion of offset adjustment via the control signal communicationcircuit 297 to the optical transmitter 1. Upon receiving the controlsignal via the control signal communication circuit 111, the opticaltransmitter 1 causes the modulation state control circuit 110 to controlthe modulator driving circuit 102 to turn on modulation of the mainsignal.

The most important advantages of this embodiment are the ability toeasily detect the optical carrier frequency even when the opticalmodulated signal spectrum is asymmetric, and the ability to match theoptical carrier frequency with the peak or bottom of the opticalfrequency characteristic of the Mach-Zehnder interferometer. In FIG. 30,the case of application to the nineteenth embodiment, based on the firstembodiment, is explained; but application to configurations based onother embodiments is also possible.

In the above, embodiments of the present invention have been explainedreferring to the drawings. However, these embodiments are merelyillustrations of the present invention, and the present inventionclearly is not limited to these embodiments. Hence various additions,omissions, substitutions, and other alterations may be made, insofar asthere is no deviation from the spirit or scope of the present invention.

INDUSTRIAL APPLICABILITY

This invention relates to an optical transmission system, and to anoptical transmitter and optical receiver for an optical transmissionsystem, to which a DPSK-DD scheme is applied; the phase difference inthe signal light of the two arms of a Mach-Zehnder interferometerprovided in the optical receiver is modulated at a constant frequency,and the phase of the component at that frequency is detected. As aresult, it is possible to set the optimum operating point of theMach-Zehnder interferometer matching the optical frequency of thetransmitting-side light source, so that optimal photo-detectioncharacteristics can be obtained.

1. An optical transmission system comprising: an optical transmitterwhich outputs differential-encoded phase-modulated light; and an opticalreceiver which detects the phase-modulated light and performsdemodulation, wherein the optical transmitter comprises: an encoderwhich converts NRZ code input signals into NRZ-I code signals; and aphase modulator which, for marks and spaces encoded by the encoder,outputs phase-modulated light with a phase deviation Δφ imparted over arange 0≦Δφ≦π, the optical receiver comprises: a Mach-Zehnderinterferometer with two independent phase adjustment terminals to set aphase difference between two interfering signals, which splits thephase-modulated light which has been received into two signal lightbeams, delays one of the split signal light beams by one bit, and causesthe two signal light beams to interfere to effect conversion intointensity-modulated light; a balanced detection circuit which performsphotoelectric conversion of signal light from two output ports of theMach-Zehnder interferometer, and outputs a difference in convertedelectrical signals; a low-frequency signal generation circuit whichapplies a first low-frequency signal at frequency f1 to one of the twophase adjustment terminals of the Mach-Zehnder interferometer; aninfinitesimal-modulated signal component detection circuit which detectsa second low-frequency signal from a signal supplied by the balanceddetection circuit; a synchronous detection circuit which, by synchronousdetection of the second low-frequency signal output from theinfinitesimal-modulated signal component detection circuit using thefirst low-frequency signal output from the low-frequency signalgeneration circuit, detects a shift amount and direction of shiftbetween a center wavelength of the phase-modulated light output from theoptical transmitter and a pass band wavelength of the Mach-Zehnderinterferometer; a control circuit which outputs a feedback error signalas a control signal to adjust the phase difference between the two splitsignal light beams so as to correct the shift amount; and an offsetsetting circuit which accents an output from the synchronous detectioncircuit and provides a signal to the control circuit; and a drivercircuit which applies the feedback error signal to the other of the twophase adjustment terminals.
 2. An optical transmission systemcomprising: an optical transmitter which outputs differential-encodedphase-modulated light; and an optical receiver which detects thephase-modulated light and performs demodulation, wherein the opticaltransmitter comprises: an encoder which converts NRZ code input signalsinto NRZ-I code signals; and a phase modulator which, for marks andspaces encoded by the encoder, outputs phase-modulated light with aphase deviation Δφ imparted over a range 0≦Δφ≦π, the optical receivercomprises: a Mach-Zehnder interferometer with phase adjustment terminalto set a phase difference between two interfering signals, which splitsthe phase-modulated light which has been received into two signal lightbeams, delays one of the split signal light beams by one bit, and causesthe two signal light beams to interfere to effect conversion intointensity-modulated light; a balanced detection circuit which performsphotoelectric conversion of signal light from two output ports of theMach-Zehnder interferometer, and outputs a difference in convertedelectrical signals; a low-frequency signal generation circuit whichapplies a first low-frequency signal at frequency f1 to the phaseadjustment terminal of the Mach-Zehnder interferometer; aninfinitesimal-modulated signal component detection circuit which detectsa second low-frequency signal from a signal supplied by the balanceddetection circuit; a synchronous detection circuit which, by synchronousdetection of the second low-frequency signal output from theinfinitesimal-modulated signal component detection circuit using thefirst low-frequency signal output from the low-frequency signalgeneration circuit, detects a shift amount and direction of shiftbetween a center wavelength of the phase-modulated light output from theoptical transmitter and a pass band wavelength of the Mach-Zehnderinterferometer; a control circuit which outputs a control signal toadjust the phase difference between the two split signal light beams soas to correct the shift amount; a driver circuit which drives the phaseadjustment terminal based on the control signal; and an optical carrierfrequency detection unit which detects, from received signal lightdetected by the balanced detection circuit, a relative position betweenan optical carrier frequency and an optical frequency characteristic ofthe Mach-Zehnder interferometer based on the frequencies correspondingto minima in the optical spectrum which are found by scanning thereceived signal light; and an offset setting circuit which accentsoutputs from the synchronous detection circuit and the optical carrierfrequency detection unit, and provides an offset to a feedback errorsignal in the control circuit, a value of the offset of the offsetsetting circuit is adjusted such that the position of the opticalcarrier frequency matches a peak position or bottom position of theoptical frequency characteristic of the Mach-Zehnder interferometer. 3.An optical transmission system comprising: an optical transmitter whichoutputs differential-encoded phase-modulated light; and an opticalreceiver which detects the phase-modulated light and performsdemodulation, wherein the optical transmitter comprises: an encoderwhich converts NRZ code input signals into NRZ-I code signals; a phasemodulator which, for marks and spaces encoded by the encoder, outputsphase-modulated light with a phase deviation Δφ imparted over a range0≦Δφ≦π; a modulation state control unit which turns on and offmodulation of a main signal; and a first control signal communicationunit which communicates with the optical receiver using a control lineprovided separately from a line for the main signal, the opticalreceiver comprises: a Mach-Zehnder interferometer with phase adjustmentterminal to set a phase difference between two interfering signals,which splits the phase-modulated light which has been received into twosignal light beams, delays one of the split signal light beams by onebit, and causes the two signal light beams to interfere to effectconversion into intensity-modulated light; a balanced detection circuitwhich performs photoelectric conversion of signal light from two outputports of the Mach-Zehnder interferometer, and outputs a difference inconverted electrical signals; a low-frequency signal generation circuitwhich applies a first low-frequency signal at frequency f1 to the phaseadjustment terminal of the Mach-Zehnder interferometer; aninfinitesimal-modulated signal component detection circuit which detectsa second low-frequency signal from a signal supplied by the balanceddetection circuit; a synchronous detection circuit which, by synchronousdetection of the second low-frequency signal output from theinfinitesimal-modulated signal component detection circuit using thefirst low-frequency signal output from the low-frequency signalgeneration circuit, detects a shift amount and direction of shiftbetween a center wavelength of the phase-modulated light output from theoptical transmitter and a pass band wavelength of the Mach-Zehnderinterferometer; a control circuit which outputs a control signal toadjust the phase difference between the two split signal light beams soas to correct the shift amount; a driver circuit which drives the phaseadjustment terminal based on the control signal; an optical carrierfrequency detection unit which detects, from received signal lightdetected by the balanced detection circuit, a relative position betweenan optical carrier frequency and an optical frequency characteristic ofthe Mach-Zehnder interferometer; an offset setting circuit whichprovides an offset to a feedback error signal in the control circuit;and a second control signal communication unit which communicates withthe optical transmitter using the control line, at the time of startupof the optical transmission system, the optical transmitter uses themodulation state control unit to turn off modulation of the main signaland transmit only an optical carrier, the optical receiver uses theoptical carrier frequency detection unit to detect the relative positionbetween the frequency of the optical carrier transmitted from theoptical transmitter and the optical frequency characteristic of theMach-Zehnder interferometer, and adjusts the offset of the offsetsetting circuit so as to cause a position of the optical carrierfrequency to match a peak or bottom position of the optical frequencycharacteristic of the Mach-Zehnder interferometer, the optical receiversends a control signal indicating completion of offset adjustment to theoptical transmitter using the second control signal communication unit,and, after receiving the control signal, the optical transmitter turnson modulation of the main signal.
 4. An optical receiver, in an opticaltransmission system comprising: an optical transmitter which outputsdifferential-encoded, phase-modulated light; and the optical receiverwhich detects the phase-modulated light and performs demodulation,wherein the optical transmitter comprises: an encoder which converts NRZcode input signals into NRZ-I code signals; and a phase modulator which,for marks and spaces encoded by the encoder, outputs phase-modulatedlight with a phase deviation Δφ imparted over the range 0≦Δφπ, theoptical receiver comprising: a Mach-Zehnder interferometer with twoindependent phase adjustment terminals to set a phase difference betweentwo interfering signals, which splits the phase-modulated light whichhas been received into two signal light beams, delays one of the splitsignal light beams by one bit, and causes the two signal light beams tointerfere to effect conversion into intensity-modulated light; abalanced detection circuit which performs photoelectric conversion ofsignal light from two output ports of the Mach-Zehnder interferometer,and outputs a difference in converted electrical signals; alow-frequency signal generation circuit which applies a firstlow-frequency signal at frequency f1 to one of the two phase adjustmentterminals of the Mach-Zehnder interferometer; an infinitesimal-modulatedsignal component detection circuit which detects a second low-frequencysignal from a signal supplied by the balanced detection circuit; asynchronous detection circuit which detects a shift amount and directionof shift between a center wavelength of the phase-modulated light outputfrom the optical transmitter and a pass band wavelength of theMach-Zehnder interferometer, through synchronous detection of the secondlow-frequency signal output from the infinitesimal-modulated signalcomponent detection circuit using the first low-frequency signal outputfrom the low-frequency signal generation circuit; a control circuitwhich outputs a feedback error signal as a control signal to adjust thephase difference between the two split signal light beams so as tocorrect the shift amount; and an offset setting circuit which accepts anoutput from the synchronous detection circuit and provides a signal tothe control circuit; and a driver circuit which applies the feedbackerror signal to the other of the two phase adjustment terminals.
 5. Anoptical receiver, in an optical transmission system comprising: anoptical transmitter which outputs differential-encoded, phase-modulatedlight; and the optical receiver which detects the phase-modulated lightand performs demodulation, wherein the optical transmitter comprises: anencoder which converts NRZ code input signals into NRZ-I code signals;and a phase modulator which, for marks and spaces encoded by theencoder, outputs phase-modulated light with a phase deviation Δφimparted over the range 0≦Δφ≦π, the optical receiver comprising: aMach-Zehnder interferometer with phase adjustment terminal to set aphase difference between two interfering signals, which splits thephase-modulated light which has been received into two signal lightbeams, delays one of the split signal light beams by one bit, and causesthe two signal light beams to interfere to effect conversion intointensity-modulated light; a balanced detection circuit which performsphotoelectric conversion of signal light from two output ports of theMach-Zehnder interferometer, and outputs a difference in convertedelectrical signals; a low-frequency signal generation circuit whichapplies a first low-frequency signal at frequency f1 to the phaseadjustment terminal of the Mach-Zehnder interferometer; aninfinitesimal-modulated signal component detection circuit which detectsa second low-frequency signal from a signal supplied by the balanceddetection circuit; a synchronous detection circuit which detects a shiftamount and direction of shift between a center wavelength of thephase-modulated light output from the optical transmitter and a passband wavelength of the Mach-Zehnder interferometer, through synchronousdetection of the second low-frequency signal output from theinfinitesimal-modulated signal component detection circuit using thefirst low-frequency signal output from the low-frequency signalgeneration circuit; a control circuit which outputs a control signal toadjust the phase difference between the two split signal light beams soas to correct the shift amount; a driver circuit which drives the phaseadjustment terminal based on the control signal; and an optical carrierfrequency detection unit which detects, from received signal lightdetected by the balanced detection circuit, a relative position betweenan optical carrier frequency and an optical frequency characteristic ofthe Mach-Zehnder interferometer based on the frequencies correspondingto minima in the optical spectrum which are found by scanning thereceived signal light; and an offset setting circuit which acceptsoutputs from the synchronous detection circuit and the optical carrierfrequency detection unit, and provides an offset to a feedback errorsignal in the control circuit, wherein a value of the offset of theoffset setting circuit is adjusted such that the position of the opticalcarrier frequency matches a peak position or bottom position of theoptical frequency characteristic of the Mach-Zehnder interferometer.